1. Field of the Invention
The present invention relates to an optical head and a solid immersion lens which can eliminate chromatic aberration, and provide a light beam having a fine spot diameter, and also concerns a recording and reproducing apparatus using such members.
2. Description of the Background Art
In recent years, for a technique to improve the information recording density in a digital recording medium that is capable of optical accessing, a technique utilizing near field lights has been developed.
In such a technique, solid immersion lens (SIL) are often used. This technique provides an arrangement in which a solid-state medium having a high refractive index is placed in the proximity of a focal point of a converged light beam in an optical head so that the light spot in the optical head is made further smaller. In the optical head of this type, a single objective lens is used and a light converging process utilizing the light refraction phenomenon is carried out.
However, in the case of the application of a plurality of wavelength in this type of optical head, when light is converged by the single objective lens as described above, a problem of “chromatic aberration” arises in which the lens fails to focus light rays having a plurality of wavelengths on the same spot.
In order to eliminate the problem of chromatic aberration, the optical microscope, etc. adopts a microscope-use objective lens which regulates chromatic aberration in a specific range of wavelengths such as a visible area by combining a plurality of lenses.
Moreover, with respect to optical recording heads, those that can deal with two wavelengths CD (wavelength 780 nm) and DVD (wavelength 635 nm) have been proposed. In this method, by utilizing the difference in numerical apertures of the two wavelengths, lights having the two different wave lengths are focused onto respective predetermined points, by changing the shape of the objective lens or inserting a holographic element depending on the aperture positions.
However, since the above-mentioned microscope lens is constituted by a plurality of combined lenses, the weight (or mass) tends to become great. Therefore, lenses of this type are not suitable for optical heads which need to move the lens.
Moreover, the above-mentioned technique which utilizes the difference in numerical apertures of two wavelengths, cannot be applied to a case in which there is no difference in the numerical apertures or the number of wavelengths (kinds) to be used is further increased. In other words, it is difficult to widely apply this technique to optical heads that deal with a plurality of wavelengths.